In the past, the well-known Gauss-type lens has been widely used as a photographic lens. Such a lens provides a standard field of view while providing a bright image. Today, Gauss-type lenses are used for various purposes and they provide advantages such as a bright image, high optical performance, and low cost of production.
A lens for use in a single lens reflex camera requires a longer back focus than a lens for use with a compact camera. Therefore, compared with the lenses used in conventional compact cameras, Gauss-type lenses provide a slightly more narrow field of view (i.e., picture angle). As a result, in some cases, such as taking a photograph indoors, it is impossible to take a photograph that encompasses a wide-angle of view, and thus there is some dissatisfaction if a Gauss-type lens is used as the standard lens of a photographic camera.
Generally speaking, if one wishes to take a photograph that encompasses a wide-angle of view, a standard lens is replaced with a wide-angle lens. However, this is often inconvenient due to the time needed to change lenses. Therefore, there has been increasing demand for using a zoom lens which can work from a standard angle to a wide-angle. However, there is a disadvantage in that carrying a zoom lens is somewhat inconvenient because of its larger size. In addition, zoom lenses tend to be more expensive.
Furthermore, many zoom lenses as well as standard lenses and wide-angle lense are unsatisfactory due to insufficient imaging performance caused by curvature of field or distortion at peripheral field angles. In order to provide a lens having a wider field of view, a lens having negative refractive power may be provided on the object side of a Gauss-type lens system. Such a construction allows the lens to have a wide-angle of view while simultaneously providing a long back focus. In this way a Gauss-type lens can be used for the lens system of a single lens reflex camera having a wide viewing field.
Japanese Examined Patent Publication H7-18972 discloses placing a negative lens group on the object side of a Gauss-type master lens wherein the spacings between the lens elements may be varied in order to provide a zoom lens with improved imaging performance at the near end of the zooming range. However, this lens system is formed of many lens elements, and thus is somewhat expensive to produce. In addition, the field angle of this lens is somewhat narrow.
For a compact camera, Japanese Patent Publication No. 3082385 discloses a lens system which has a lens with negative refractive power positioned on the object side of the lens system. However, this lens system is not usable for a single lens reflex camera because the back focus is too short, since this lens system was designed for a compact camera.
The present invention is a photographic lens for use in a single lens reflex camera. The objects of the invention are to provide a lens of simple construction that is inexpensive to produce, to provide a high quality image, and to provide both a large back focus and a field of view that is slightly larger than the standard field of view. More specifically, the present invention is a high performance standard photographic lens wherein a lens having negative refractive power is positioned on the object side of a master lens having positive refractive power in order to provide a lens having a large back focus. Spherical aberration, curvature of field, and distortion are favorably corrected so as to provide images of high quality.